This invention relates to a moving picture decoding device in which picture data is read from a picture memory responsive to a motion vector to perform decoding of motion-compensated moving pictures and in which the picture data are also read in a pre-set sequence for display.
There are a variety of systems for compressing and encoding moving picture signals, such as television telephone/television conference signals or telecast signals. Recently, a hybrid encoding system, such as MC-DCT, which consists in a combination of a so-called motion compensated (MC) inter-frame prediction and discrete cosine transform (DCT), is thought to be promising.
FIG. 1 shows a circuit arrangement for illustrating the above-mentioned MC-DCT hybrid system. In this figure, moving picture signals, such as television signals, are supplied as input signals to an input terminal 111. These input signals are supplied to a motion detection circuit 113 and a subtractive node 114 via a picture memory 112 employed as a frame memory. An output of the subtractive node 114 is transmitted to a DCT circuit 115 for discrete cosine transformation and thence supplied to a quantizer 116 for quantization before being supplied to a series circuit as a local decoder, consisting of a inverse quantization unit 117 and an inverse DCT (IDCT) circuit 118. An output of the IDCT circuit 118 is supplied via an additive node 119 to a picture memory 120 employed as a field memory. An output read from the picture memory 120 is transmitted to the motion detection circuit 113 and to a motion compensation circuit 121. The motion detection information such as the motion vector from the motion detection circuit 113 is transmitted to the motion compensation circuit 121. An output of the motion compensation circuit 121 is supplied to the subtractive node 114 and to the additive node 119.
It is noted that the input signals are stored temporarily in the picture memory 112 and subsequently read and processed on the basis of a block of a pre-set size. The motion detection circuit 113 compares the values of pixels of a signal block from the picture memory 112 to the values of pixels of locally decoded signals from the picture memory 120 for detecting the motion vector. The motion compensation circuit 121 outputs a reference block to the subtractive node 114 based on this motion vector. The subtractive node 114 outputs a difference between the input picture signal block and the reference block. The difference output is discrete cosine transformed by the DCT circuit 115 and quantized by the quantizer 116 before being supplied to a variable length coding unit 123, such as an entropy coding unit, for variable length coding. The motion vector from the motion detection circuit 113 is also supplied to the variable length coding unit 123 for variable length coding.
An output of the variable length coding unit 123 is supplied to a transmitting buffer memory 125 where the coded data to be transmitted is stored transiently. The quantization by the quantizer 116 and the coding by the variable length coding unit 123 are controlled so that the amount of transmitted data per unit time will be constant. An output of the buffer memory 125 is outputted via an output terminal 126 so as to be transmitted over a communication network or recorded/reproduced on or from recording medium.
If the input signals are color component picture signals, made up of Y (luminance) signals and C (chroma) signals, the MC and DCT operations are performed on both the Y signal data and the C signal data. The C signals are made up of color difference signals C.sub.b and C.sub.r corresponding to so-called B-Y signals and R-Y signals, respectively. As for the numbers of samples or the sampling frequency, the ratio of Y:C.sub.b :C.sub.r is set to 4:2:2, such that one C.sub.b pixel data and one C.sub.r pixel data are associated with two Y pixel data.
In decoding the signals, processed with the above-described MC-DCT hybrid coding operations, it is necessary to read data of a frame directly preceding the current frame from the frame memory in accordance with the motion vector to perform motion compensation thereon. On the other hand, in displaying the signals on a display unit, such as a cathode ray tube (CRT) monitor, it is necessary to read the data sequentially from the memory in accordance with the scanning operation for display.
The frame memory is made up of a number of, such as four, memory devices, such as DRAMs, and is adapted for reading out data from the memory devices by parallel reading with four bytes, as an example, as a word, at a rate of one byte from each memory device.
In accessing the data on the frame memory, such a word format may be contemplated in which two bytes, for example, of Y data and each one byte of the C.sub.b and C.sub.r data, totalling at four bytes, make up each word. Such word format dispenses with a buffer memory for display, However, a problem is raised that the buffer memory for adjusting the timing when summing the motion-compensated picture data to the inter-frame difference data is increased in capacity, On the other hand, if a word format convenient for MC processing such as a word format in which a word consisting only of four Y bytes is changed over to a word consisting only of four C bytes or vice versa as time elapses, is employed, it becomes necessary to provide a buffer memory for display while the number of times of data reading from the frame memory for MC processing is increased, even though the buffer memory for timing adjustment for MC processed data may be reduced in capacity.